1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (OFDM) wireless communication apparatus and method.
2. Description of the Related Art
In recent years, there have been widely used communication instruments equipped with OFDM (Orthogonal Frequency Division Multiplexing) modulation technologies, such as a wireless LAN (Local Area Network) represented by IEEE 802.11a/g, and IEEE 802.16 or WiMax. Most of the communication instruments are mounted on battery-driven terminals, so that there is a need to minimize power consumption while ensuring stable communication quality and high throughput.
To meet such conflicting requirements, attention is given to adaptive control of transmission power. In the most simplified example, a pair of transmitting and receiving terminals communicate with each other using low power when they are within a short transfer distance, and necessary transmission power will be added as the distance between them increases, so as to minimize the transmission power.
Multipath resulting from various reflective objects, such as walls and ground, is present on an actual wireless transmission line, resulting in frequency selective fading. In FIG. 9 showing a specific example, when transmission power is sent at a uniform power density for a certain frequency band, a large number of delayed waves due to the multipath impair the uniformity of the power density in the reception spectrum.
Consider now how the fading phenomenon affects the OFDM. Since the OFDM is formed of a large number of narrowband carriers (hereinafter referred to as subcarriers), the amount of signal attenuation of one subcarrier will differ from those of other subcarriers, resulting in subcarrier-to-subcarrier variance in error characteristic. Accordingly, it will be difficult to keep an error-free state for all subcarriers.
To increase the reception sensitivity, any one of the following actions is necessary: adding extra transmission power, reselecting more error-resistant modulation method, or adjusting the error coding rate. However, any one of the above actions forces increased power consumption or reduced frequency efficiency.
JP-A-2005-45505 proposes a method in which transmission power for each subcarrier is increased or decreased on the transmitting side such that variation in reception level for each subcarrier is reduced on the receiving side, and when such a control does not provide a desired reception quality, extra uniform transmission power will be added for all transmission power.
JP-A-2001-186102 proposes a method in which in multicarrier communication, higher transmission power and more transmission bits are allocated to a carrier with less noise, while transmission power is reduced for a carrier with more noise, so as to reduce the number of the transmission bits.
JP-A-2004-266585 proposes a method in which in multicarrier communication, higher transmission power is allocated to a carrier having a higher S/N ratio to keep the sum of the signal power and the noise power constant based on the water filling theorem, while lower power is allocated to a carrier having a lower S/N ratio. In JP-A-2004-266585, the optimum power distribution is carried out based on the same theorem along the time axis as well as the frequency axis.
The common feature in JP-A-2001-186102 and JP-A-2004-266585 is that the number of transmittable bits for all subchannels is maximized by distributing transmission power in an optimum manner and maximizing the total channel capacity of all subcarriers. That is, even when the total transmission power is constant, the methods described in JP-A-2001-186102 and JP-A-2004-266585 are used to perform optimizing control of transmission power distribution per unit frequency and unit time, so as to increase the throughput.